decays very approximately according to an exponential law with the time, falling to half value in about 3 minutes. Three or four hours after removal the activity again decays according to an exponential law with the time, falling to half value in about 28 minutes. The family of curves obtained for different times of exposure have already been shown in Fig. 67. These results thus indicate:—
(1) An initial change in which half the matter is transformed in 3 minutes;
(2) A final change in which half the matter is transformed in 28 minutes.
Before considering the explanation of the intermediate portion of the curve further experimental results will be considered.
The